The field of integrated circuits and electronic devices is ever-expanding. As the field expands, the integrated circuits and electronic devices themselves become smaller and faster. The decreasing size and increasing speed provides a challenge to equipment designers and manufacturers in that a large number of signals which occur at very high frequencies must be precisely coordinated. In order to ensure reliable operation, functionality and timing tests must be performed on integrated circuits and electronic devices.
To that end, a wide variety of tools and techniques have been developed in the field of testing. One such technique, referred to as direct time measurement, utilizes a time measurement device to detect certain signal events occurring within a measured signal. A signal event, as referenced herein, is intended to represent the point in a signal at which the signal voltage transitions above or below a given threshold voltage. These time measurement devices typically feature a number of measurement channels, each of which contains at least one measurement circuit. An example of such a device can be found in U.S. Pat. No. 6,091,671, issued to Kattan, for a time interval analyzer, which is incorporated by reference for all purposes herein. Another example of a time measurement device is found in U.S. Pat. No. 6,194,925, issued to Kimsal et al. and herein fully incorporated for all purposes by reference. The ""925 patent discloses a time interval measurement system in which a voltage differential across a hold capacitor generated between events occurring in an input signal determines the time interval between events. The measurement system of the ""925 patent utilizes a linear ramp generating circuit to ensure a linear discharge of the capacitor for easier measurement of the occurrence of the events.
Still another suitable time measurement device that may be used with aspects of the present technology is found in U.S. Pat. No. 4,757,452, issued to Scott, et al., and herein fully incorporated for all purposes by reference. The ""452 patent provides a system for measuring timing jitter of a tributary data stream that has been multiplexed into a higher-rate multiplex stream using pulse stuffing techniques. The ""452 patent is an event-counter-based system that does not directly measure time intervals but determines their frequency by maintaining a continuous count of the number of pulses occurring within a signal.
A wide variety of measurement circuits are available, many of which detect an event using a comparator. A signal event is typically detected by comparing the voltage of the measurement signal with a direct current (DC) threshold voltage, thereby converting the measured signal into a more readily processed timing signal. A comparator, as is known in the art, compares one voltage to another, and produces a signal indicating which voltage is greater or lesser (depending upon the configuration). In a typical comparator, one voltage is designated the reference (or threshold) voltage. When the other voltage input exceeds the threshold voltage, the output shifts from low to high (or high to low, again depending upon configuration). Thus, a measurement circuit comparator typically receives a signal to be measured and converts it to a binary timing signal. This timing signal may then be routed to other components in a measurement circuit, where it is more readily processed for obtaining certain timing information.
While precision components are preferably used in the construction of the internal circuitry measurement circuits, there is a certain margin of error that is unavoidable. This may be due to unmatched components, ambient noise, temperature effects, or irregularities in fabrication. Such a margin exists in the threshold voltage-generating circuitry, and consequently there is a need to ensure that this voltage level is properly set.
Known methods of ensuring the accuracy of a threshold voltage may involve inputting a DC calibration voltage to compare to the threshold voltage and adjusting the threshold voltage level accordingly. The calibration voltages, however, are typically pure DC values. This leaves open the possibility for distortions in the calibration signal, as well as error in the calibration process since pure DC voltages can only be applied at stepped levels. The xe2x80x9cproperxe2x80x9d level may very well be missed depending upon size of the step between levels.
Say, for example, a threshold voltage of 1 Volt is selected for a comparator in a time measurement device. Unbeknownst to the user, there is an offset of 0.005 Volts. Consequently, a setting of 1 Volt results in a VTH of 1.005 Volts. For a true threshold voltage of 1 Volt, the threshold should be set at 0.995 Volts.
To find the unknown offset, a DC calibration signal is input to the comparator with an initial value of 0.99 Volts. It is stepped in 0.01 Volt increments. At 0.99 and 1.00 Volts, nothing is detected. At 1.01 Volts, the signal is detected, because the (true) VTH of 1.005 has been exceeded. The true offset remains unknown because the step size was too large, and detection precision is subsequently lost when VTH is adjusted. Using a smaller step size may be impractical or impossible for the extremely small offsets encountered with precision components.
In view of the recognized features encountered in the prior art and addressed by the present subject matter, an improved method for calibrating threshold voltage levels has been developed.
More particularly, dithered voltage generating circuitry to aid in calibration of a threshold voltage for use with a measurement device is disclosed. Such circuitry preferably comprises a first voltage source producing a DC output, a second voltage source producing a time-varying output, a waveform shaping circuit, and a summing circuit. The waveform-shaping circuit is electrically connected to the output of the second voltage source and modifies the time-varying output provided by such source. The summing circuit is configured to add the DC output produced by the first voltage source to the modified time-varying output produced by the second voltage source in conjunction with the waveform shaping circuit. The summing circuit outputs a calibration waveform comprising a nominal DC value featuring a plurality of peaks extending above the nominal DC value. In one particular exemplary embodiment, the waveform shaping circuit may comprise a diode, while the summing circuit may comprise a summing amplifier. The time-varying output of the second voltage source may feature smooth, rather than stepped, transitions between voltage levels. The second voltage source may comprise a function generator. The dithered voltage generating circuitry may be contained within a module connected to a comparator circuit of a measurement device such as a time interval analyzer.
In accordance with aspects of the invention, the measurement device is configured to detect, by monitoring the output of the comparator circuit, the peaks of the calibration waveform and adjust the threshold voltage until the threshold voltage is equal to the nominal DC value of the calibration waveform. Additional embodiments include a digital-to-analog converter under the control of the measurement device for producing and adjusting the threshold voltage input to the comparator circuit.
Additionally disclosed is a method for calibrating a threshold voltage input to a comparator circuit of a measurement device. Such a calibration method may comprise the steps of generating a calibration waveform comprising a nominal DC value featuring a plurality of peaks extending above the nominal DC value, providing the calibration waveform to a first input of the comparator circuit, providing a threshold voltage to a second input of the comparator circuit, and monitoring the output of the comparator circuit and changing the threshold voltage until the threshold voltage is equal to the nominal DC value of the calibration waveform. The nominal value of the calibration waveform may be the desired value for the threshold voltage provided to the comparator. In one embodiment, the threshold voltage is provided by a digital-to-analog converter in response to control commands issued from the measurement device. The calibration waveform may be generated by the summation of a DC voltage and a half-wave-rectified sinusoidal voltage, and the measurement device may comprise a time interval analyzer.
Further disclosed is a method for producing a calibration waveform to aid in the calibration of threshold voltage levels input to a comparator circuit, which includes the steps of generating a DC waveform and a time-varying voltage waveform, modifying the time-varying voltage waveform, and summing the modified and DC waveforms. Embodiments of the method include those in which the DC waveform is generated by a precision voltage source an the time-varying voltage waveform is rectified by a diode. Additional embodiments may employ a function generator to produce the time-varying voltage waveform and summing amplifier circuitry to produce the calibration waveform. In further additional embodiments, the resulting calibration waveform is output to a comparator circuit of a measurement device, which can include time interval analyzers.
In one embodiment of the disclosed method, the calibration waveform can be generated by a module that is selectively connected to comparator circuitry of a measurement device such as a time interval analyzer. A switch may connect the module to one or more comparator circuits, and the resulting determined offset voltages may be stored in an error table for use in future measurements.
Additional embodiments and aspects of the subject invention, not necessarily expressed in this summarized section, may include and incorporate various combinations of aspects of features, parts or steps referenced in the summarized description above, and/or other features, parts, steps, or elements as otherwise discussed. Those of ordinary skill in the art will better appreciate the features and aspects of such embodiments upon review of the specification hereinbelow. Thus, the scope of the presently disclosed technology should in no way be limited to any particular embodiment.